The present invention relates in general to semiconductor technology and in particular to structures and methods for forming performance enhancing PN clamps in such semiconductor structures as trench MOS barrier Schottky (TMBS) rectifiers.
Semiconductor-based power rectifiers are well known and have been used in power electronic systems for many decades. Schottky rectifiers have generally been used in applications operating at mid to low voltages due to their low on-state voltage drop and fast switching speed. Schottky rectifiers can be optimized by changing the Schottky contact metal to alter the barrier height. There is a tradeoff, however, between forward voltage drop and reverse leakage current. As the barrier height is reduced, the forward voltage drop decreases but the reverse leakage current increases. On the other hand, as the barrier height is increased, the forward voltage drop increases but the reverse leakage current decreases.
This tradeoff between forward voltage drop and reverse leakage current can be improved using a TMBS rectifier structure. The trench MOS structure of the TMBS rectifier greatly reduces the electric field under the Schottky contact thus increasing reverse breakdown voltage and reducing reverse leakage current. This allows higher doping concentration in the mesa regions thus reducing the rectifier's on-state voltage drop.
One variant of the TMBS rectifier structure includes a PN junction formed below each trench. Like the TMBS structure described above, under reverse bias the depletion regions merge to reduce leakage current. The PN junction can also improve breakdown characteristics of the Schottky contact by clamping the reverse voltage at a lower avalanche breakdown of the PN junction. Conventional methods of forming such a structure involve implanting dopants into the semiconductor region along the bottom of the trench and thermally activating the dopants in the semiconductor region. Thermal cycles from subsequent processing steps, however, can cause excessive lateral diffusion of the implanted dopants thus limiting cell pitch.
Thus, improved structures and methods for forming PN clamps in such semiconductor structures as TMBS rectifiers are desired.